In the majority of applications printing proceeds by pressure contact of an ink-loaden printing form with an ink-receiving material which is usually plain paper. The most frequently used impact printing technique is known as lithographic printing based on the selective acceptance of oleophilic ink on a suitable receptor.
In recent times however so-called non-impact printing systems have replaced classical pressure-contact printing to some extent for specific applications. A survey is given e.g. in the book “Principles of Non Impact Printing” by Jerome L. Johnson (1986), Palatino Press, Irvine, Calif. 92715, USA.
Among non-impact printing techniques ink jet printing has become a popular technique because of its simplicity, convenience and low cost. Especially in those instances where a limited edition of the printed matter is needed ink jet printing has become a technology of choice. A recent survey on progress and trends in ink jet printing technology is given by Hue P. Le in Journal of Imaging Science and Technology Vol. 42 (1), January/February 1998.
In ink jet printing tiny drops of ink fluid are projected directly onto an ink receptor surface without physical contact between the printing device and the receptor. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving the print head across the paper or vice versa. Early patents on ink jet printers include U.S. Pat. No. 3,739,393, U.S. Pat. No. 3,805,273 and U.S. Pat. No. 3,891,121.
The jetting of the ink droplets can be performed in several different ways. In a first type of process a continuous droplet stream is created by applying a pressure wave pattern. This process is known as continuous ink jet printing. In a first embodiment the droplet stream is divided into droplets that are electrostatically charged, deflected and recollected, and into droplets that remain uncharged, continue their way undeflected, and form the image. Alternatively, the charged deflected stream forms the image and the uncharged undeflected jet is recollected. In this variant of continuous ink jet printing several jets are deflected to a different degree and thus record the image (multideflection system).
According to a second process the ink droplets can be created “on demand” (“DOD” or “drop on demand” method) whereby the printing device ejects the droplets only when they are used in imaging on a receiver thereby avoiding the complexity of drop charging, deflection hardware, and ink recollection. In drop-on-demand the ink droplet can be formed by means of a pressure wave created by a mechanical motion of a piezoelectric transducer (so-called “piezo method”), or by means of discrete thermal pushes (so-called “bubble jet” method, or “thermal jet” method).
Ink compositions for ink jet typically include following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc. It will be readily understood that the optimal composition of such an ink is dependent on the ink jetting method used and on the nature of the substrate to be printed. The ink compositions can be roughly divided in:                water based; the drying mechanism involves absorption, penetration and evaporation;        oil based; the drying involves absorption and penetration;        solvent based; the drying mechanism involves primarely evaporation;        hot melt or phase change: the ink vehicle is liquid at the ejection temperature but solid at room temperature; drying is replaced by solidification;        UV-curable; drying is replaced by polymerization.        
It is known that the ink-receiving layers in ink-jet recording elements must meet different stringent requirements:                The ink-receiving layer should have a high ink absorbing capacity, so that the dots will not flow out and will not be expanded more than is necessary to obtain a high optical density.        The ink-receiving layer should have a high ink absorbing speed (short ink drying time) so that the ink droplets will not feather if smeared immediately after applying.        The ink dots that are applied to the ink-receiving layer should be substantially round in shape and smooth at their peripheries. The dot diameter must be constant and accurately controlled.        The receiving layer must be readily wetted so that there is no “puddling”, i.e. coalescence of adjacent ink dots, and an earlier absorbed ink drop should not show any “bleeding”, i.e. overlap with neighbouring or later placed dots.        Transparent ink-jet recording elements must have a low haze-value and be excellent in transmittance properties.        After being printed the image must have a good resistance regarding water-fastness, light-fastness, and good endurance under severe conditions of temperature and humidity.        The ink jet recording element may not show any curl or sticky behaviour if stacked before or after being printed.        The ink jet recording element must be able to move smoothly through different types of printers.        
All these properties are often in a relation of trade-off. It is difficult to satisfy them all at the same time.
It is known that the presence in the ink accepting layer of absorptive pigments such as silica, kaolin, talc, aluminum oxide, boehmite, etc. improve the absorption capacity, the obtainable colour density and the drying time. Many patent applications have described this effect for many different binder-systems. U.S. Pat. No. 3,357,846 describes pigments such as kaolin, talc, bariet, TiO2 used in starch and PVA. U.S. Pat. No. 3,889,270 describes silica in gelatin, PVA and cellulose. Pigments and particles have also been described in patent applications including DE 2,925,769, GB 2,050,866, U.S. Pat. No. 4,474,850, U.S. Pat. No. 4,547,405, U.S. Pat. No. 4,578,285, WO 88 06532, U.S. Pat. No. 4,849,286, EP 339604, EP 400681, EP 407881, EP 411638 and U.S. Pat. No. 5,045,864 (non-exhaustive list).
These particulates are dispersed in various types of binders of which the most common types such as gelatin, polyvinyl alcohol, polyvinyl pyrrolidone, and various types of cellulose derivatives. These conventional binders are mentioned in numerous patent documents.
It is further known to use various types of film-forming polymers to improve the waterfastness of the coated ink receiving layer by increasing its adhesion to the support.
It is also known to improve the waterfastness of the finished printed image by the incorporation of mordanting polymers bearing cationic quaternary groups such as ammonium-, sulphonium and phosphonium groups. These polymers interact with most typical ink jet inks resulting in a better anchoring of the dye in the binder. Typical examples of such descriptions are U.S. Pat. No. 4,371,582, U.S. Pat. No. 4,575,465, U.S. Pat. No. 4,649,064, GB 2 210 071 and EP 423829.
Since the present invention is particularly directed to an improved recording material for outdoor use, such as billboards, banners, signs and advertisement pannels, it will be readily understood that the so-called weatherability characteristics of such a material are of prior importance. These necessary characteristics include excellent water-fastness, light-fastness, and resistance to high and low temperature.
It is well known to those skilled in the art that the use of inks containing pigments gives superior ink jet images when light-fastness is concerned. So printing with pigment inks is preferred when the final image is meant for outdoor use. However, such images suffer from a lack of water-fastness, since the pigments remain partially near the surface of the medium and can be partially smeared out in wet condition by a certain mechanical force, e.g. a wet finger stroke. So, there is a permanent need for improvement for water-fastness for this type of materials.
The present invention extends the teachings on ink jet recording materials and is particularly directed to to an improved material for outdoor use.